1. Field of the Invention
The invention relates to a control apparatus for a power train including a continuously variable transmission in which a torque transmitting member, such as a belt or a power roller, for transmitting torque is in direct or indirect contact with rotary members, such as pulleys or discs, and which has a variable torque capacity that varies depending upon a contact pressure between the torque transmitting member and the rotary members. In particular, the invention relates to such a control apparatus adapted for controlling an engaging pressure of a clutch that is arranged in series with the continuously variable transmission.
2. Description of Related Art
Known continuously variable transmissions (hereinafter referred to as “CVT” when appropriate) is constructed so as to change the speed ratio (or transmission ratio) continuously or steplessly, by continuously changing contact positions or torque transmitting positions between a torque transmitting member, such as a belt or a power roller, and rotary members, such as a pulleys or a discs. The torque transmission is effected by utilizing frictional force or shearing force of traction oil. The torque capacity of the continuously variable transmission is determined based on a contact pressure between the torque transmitting member and the pulley or disc or a pressure (i.e., clamping pressure) for clamping or pinching the torque transmitting member, and the coefficient of friction or the shearing force of the traction oil. If a torque applied to the CVT exceeds the torque capacity, a slip of the belt or power roller occurs.
If an excessive slip of the belt or power roller occurs, a portion of the pulley or disc that is in contact with the belt or power roller wears. As a result, the worn portion of the pulley or disc will not serve to transmit torque, and the CVT will not be able to perform its intended function of transmitting power at a given speed ratio. In order to prevent a slip of the CVT during running of the vehicle on which the CVT is installed, it has been proposed to increase the clamping or pinching force applied to the torque transmitting member, thereby to increase the torque capacity.
If the clamping pressure is increased, however, the power transmitting efficiency of the CVT is reduced, and a large quantity of power is consumed for driving an oil pump that generates a hydraulic pressure, resulting in deteriorated fuel economy of the vehicle. It is thus preferable to reduce the clamping pressure of the CVT to the minimum within a range in which no slip occurs.
If the vehicle is in a non-steady running state in which the output torque of the engine or the negative torque applied from vehicle wheels changes frequently or largely, the torque applied to the CVT cannot be predicted, and therefore the safety factor or an excess of the torque capacity (i.e., an excess amount of the torque capacity over the minimum or limit torque capacity with which no slip occurs in a steady running state) needs to be increased so as to establish a relatively high clamping pressure. If the vehicle is in a steady or quasi-steady running state, on the other hand, the torque applied to the CVT is stable, and therefore the clamping pressure can be lowered until the CVT is brought into an operating state immediately before a slip occurs.
However, an unexpected or sudden torque may occur even in a steady or quasi-steady running state, which makes it necessary to prevent or avoid a slip of the CVT even in this case. To meet with this requirement, it has been proposed in, for example, Japanese laid-open Patent Publication No. 10-2930 to provide a clutch in series with the CVT, and set an excess of the engaging pressure of the clutch smaller than an excess of the clamping pressure of the CVT. With this arrangement, the engaging pressure of the clutch and the clamping pressure of the CVT are reduced when no slip of the clutch is detected, and the engaging pressure and the clamping pressure are both controlled to be increased when a slip of the clutch is detected. Here, the excess of the engaging pressure or clamping pressure is an excess amount of the engaging pressure or clamping pressure over the minimum or limit level at which no slip occurs in a steady running state of the vehicle.
With the above-described control, when a torque applied to the power train in which the clutch and the CVT are arranged in series increases, the clutch is caused to slip first, namely, before a slip of the CVT occurs, so as to limit the torque applied to the CVT, thereby avoiding in advance slippage of the CVT. In other words, the clutch arranged in series with the CVT functions as a so-called “torque fuse”.
In the control device as disclosed in the above-identified publication, the engaging pressure of the clutch and the clamping pressure of the CVT are reduced when no slip of the clutch is detected, and are increased when a slip of the clutch is detected as a result of reduction of the engaging pressure. In this manner, the engaging pressure and the clamping pressure are repeatedly reduced and increased, resulting in repeated slips of the clutch. With the known control device as described above, therefore, the power transmitting efficiency of the power train may be reduced, and the fuel economy may be deteriorated or may not be improved owing to the use of the CVT.
It is also to be noted that control of the engaging pressure of the clutch involves inevitable variations because of differences among individual clutches in terms of characteristics of a hydraulic control device associated with the clutch and/or the coefficient of friction of the clutch. Since the known device as disclosed in the above-identified publication does not take these variations into consideration when increasing the engaging pressure and the clamping pressure after detection of a slip of the clutch, the engaging pressure of the clutch may become excessively large relative to the clamping pressure. In such a case, an excess of the engaging pressure of the clutch may become substantially equal to or larger than that of the clamping pressure of the CVT, which may result in occurrence a slip of the CVT.